The properties of defects in organic-inorganic hybrid perovskite are widely studied from the first-principles calculation. However, the defects of methylamine (methylamine = CH3NH2), which would be easily formed during the preparation of the organic-inorganic hybrid perovskite, are rarely investigated. Thermodynamic properties as well as defect states of methylamine embedded MAPbX3 (MA = methyl-ammonium = CH3NH3, X = Br, I) are studied based on first-principles calculations of density functional theory. It was found that there is a shallow defect level near the highest occupied molecular orbital, which induced by the interstitial methylamine defect in MAPbBr3, will lead to an increase of photoluminescence. The calculation results showed that interstitial defect states of methylamine may move deeper due to the interaction between methylamine molecules and methyl-ammonium cations. It was also showed that the interstitial methylamine defect is stable at room temperature, and the defect can be removed easily by annealing.
As an alternative technology to the standard evaporated metal top electrode in perovskite solar cells, the transfer method top electrode is featured with low cost, simple process, and ease of scalability. However, as the transfer method preparation of the less expensive metal electrodes (Ag and Cu) is explored, the mismatch between the hole‐transport layer and the top electrode results in anomalous photovoltaic properties. In this study, the work function of the hole‐transporting material is adjusted by changing the dopant content to achieve a good energy‐level match with either Ag or Cu electrode material. The power conversion efficiencies of perovskite devices based on transferred Ag and Cu electrodes can reach 18.41% and 17.25%, respectively, which are comparable to the performance of the evaporated top electrode devices.
The halide perovskite solar cells employing CH3NH3PbX3 (X=Cl-, Br-, I-) and CH3NH3PbI3-xClx as light absorbers each have shown a rapid rise in power conversion efficiency (PCE) from 3.8% to 22.1% in recent years. The excellent photovoltaic performance is attributed to good optical and electrical properties such as appropriate bandgap, large absorption coefficient, high carrier mobility, long carrier lifetime and long carrier diffusion length. However, the physical mechanism of high PCE for halide perovskite solar cells is still unclear. The Gaussian 09 software is utilized to optimize the geometries of isolated CH3NH3+ and CH3NH3 at a B3 LYP/6-311++G(d, p) level, and the Multiwfn software is used to visualize the electrostatic potentials (ESPs) of CH3NH3+ and CH3NH3. Based on the ESPs of CH3NH3+ and CH3NH3, it is found that the CH3NH3+ has a strong electrophilic character, however, the NH3- side and CH3- side of CH3NH3 have weak nucleophilic and electrophilic character, respectively. So the electrostatic characteristics of CH3NH3+ and CH3NH3 are significantly different. The strong electrostatic repulsive interaction between two neighboring CH3NH3+ radicals plays an important role in structural phase transition of CH3NH3PbI3 material. At room temperature, the CH3NH3+ in the inorganic cage is activated and disordered, and has a strong electrophilic character. Due to these characteristics of CH3NH3+, the interfacial electrons at TiO2/CH3NH3PbI3 heterojunction are combined with CH3NH3+ to form CH3NH3 in the inorganic[PbI3]- framework. The CH3NH3 at the heterojunction under the built-in electric field is more easily oriented than CH3NH3+. Two initial geometrical configurations for CH3NH3+:CH3NH3 and CH3NH3:CH3NH3 dimers are optimized by using Gaussian 09 at an MP2/Aug-cc-PVTZ level. On the basis of the electrostatic characteristic of CH3NH3+:CH3NH3 dimer, the interfacial electrons at TiO2/CH3NH3PbI3 heterojunction are easily injected into the CH3NH3PbI3 material, which leads to the strong polarization of CH3NH3PbI3 material at the heterojunction. From the ESP of optimized CH3NH3:CH3NH3 dimer, it is found that the weak electrostatic field of the inorganic framework, parallel to C-N axis, is induced by the CH3NH3 orientational order, which is made for improving the photogenerated electron-hole pair separation and carrier transport. The TiO2/CH3NH3PbI3 heterojunction has more advantage than traditional p-n junction because of no consumption of carrier for CH3NH3PbI3 material in the process of forming built-in electric field. The physical mechanism is the origin of high PCE for CH3NH3PbI3 solar cells. According to the experimental results and first-principle calculations, we can draw an important conclusion that the electrostatic characteristics of organic CH3NH3+ cations in the inorganic[PbI3]- framework result in the high performances of halide perovskite solar cells.
CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> is one of the most promising candidates for high-performance hybrid organic-inorganic perovskite solar cells. The CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> single crystal and polycrystalline thin film exhibit the unique features of long carrier lifetimes and diffusion lengths, however, their carrier mobilities are in fact rather modest in a range from 1 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> to 100 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. Experimentally, the temperature dependence of mobility is described as T<sup>–1.3</sup> to T<sup>–1.6</sup> due to the acoustic phonon scattering. To be sure, the rotating CH<sub>3</sub>NH<inline-formula><tex-math id="Z-20210812103905">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103905.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103905.png"/></alternatives></inline-formula> cations are disadvantageous to the carrier transport and performance for CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells. The effect of the rotating CH<sub>3</sub>NH<inline-formula><tex-math id="Z-20210812103911">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103911.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103911.png"/></alternatives></inline-formula> cations on high-performance CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells remains an open question. The Gaussian 09 software has been utilized to optimize the geometrical structures of CH<sub>3</sub>NH<sub>3</sub> dimer, trimer, tetramer, and pentamer in isolated state at the MP2 level with using the cc-PVTZ basis set. For CH<sub>3</sub>NH<sub>3</sub> polymer, the mean distance between two centroids of neighboring CH<sub>3</sub>NH<sub>3</sub> decreasing with the number of CH<sub>3</sub>NH<sub>3</sub> is slightly smaller than the lattice constant 6.28 Å of tetragonal CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, which is advantageous to structural stability and higher structural order of inorganic [PbI3]<sup>–</sup> framework. It signifies that the long range order of electrically neutral CH<sub>3</sub>NH<sub>3</sub> is easily formed for room-temperature CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>. The total dipole moment linearly increases with the number of CH<sub>3</sub>NH<sub>3</sub> for CH<sub>3</sub>NH<sub>3</sub> polymer, and attains a large value 19.7 Debye for CH<sub>3</sub>NH<sub>3</sub> pentamer, which may be the origin of strong polarization in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> heterojunction. The molecular orbitals of five unpaired electrons for CH<sub>3</sub>NH<sub>3</sub> pentamer are distributed around NH<sub>3</sub>-sides of five different CH<sub>3</sub>NH<sub>3</sub> pentamers respectively, and these orbital energies are in a range from –4.4 eV to –3.2 eV. The unpaired electrons in CH<sub>3</sub>NH<sub>3</sub> polymer have an electrostatic attraction on the CH<sub>3</sub>-side of neighboring CH<sub>3</sub>NH<sub>3</sub>, which is the key cause of forming the ordered CH<sub>3</sub>NH<sub>3</sub> polymer. Hence it can be inferred that the orbital energies of unpaired electrons are getting closer when the longer range order of CH<sub>3</sub>NH<sub>3</sub> are formed in room-temperature CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> through the interfacial electron injection. The vector field map of electrostatic potential (ESP) shows that CH<sub>3</sub>NH<inline-formula><tex-math id="Z-20210812103926">\begin{document}${}_3^+ $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103926.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210353_Z-20210812103926.png"/></alternatives></inline-formula> has strong electrophilic character, and the NH<sub>3</sub>-side has a stronger electrophilic character than CH<sub>3</sub>-side, however, CH<sub>3</sub>NH<sub>3</sub> monomer and polymer have weak electrophilic and nucleophilic character. Thus, the forming of CH<sub>3</sub>NH<sub>3</sub> polymer at the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> heterojunction leads the organic and inorganic portions to be decoupled, which can effectively reduce the anharmonic phonon modes. Under an applied electric field, the unpaired electrons in CH<sub>3</sub>NH<sub>3</sub> pentamer can transfer along the C-N axis through the hopping mechanism. According to these results, we can draw three useful conclusions below. i) The electrons under an applied electric field are easily injected into the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> material through the heterojunction, the CH<sub>3</sub>NH<sub>3</sub> polymer is easily formed, and the unpaired electrons in polymer are transferred between two neighboring CH<sub>3</sub>NH<sub>3</sub> through hopping mechanism. ii) The decoupling between organic CH<sub>3</sub>NH<sub>3</sub> and inorganic [PbI3]<sup>–</sup> framework can effectively reduce the anharmonic phonon modes, which can lead the carrier scattering decrease and the efficiency of carrier separation and transport to improve; iii) The ordered CH<sub>3</sub>NH<sub>3</sub> polymer at the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> heterojunction can enhance the order of inorganic [PbI3]<sup>–</sup> framework. Our researches may help to further understand the origin of high power conversion efficiency (PCE) for hybrid organic-inorganic perovskite solar cells.
As an alternative of evaporated metal top electrode in perovskite solar cells, the transferred top electrode technique has the outstanding advantage of simple process. And in the case of carbon based transferred top electrode, the efficiency of corresponding perovskite devices can be higher than 20%. During the solid-to-solid electrode transferring process, it is essential to achieve effective contact and to avoid damage of bottom functional layers for an improved device performance. In this work, a sprayed graphite layer on conductive substrate is employed as transferred top electrode for perovskite solar cells. A simple mechanical polishing treatment is further adopted to modify the surface status of graphite layer. Compared with as sprayed graphite electrode, polished electrode shows better electrical property and optimized interface contact with hole-transporting layer. Thus, the average efficiency of perovskite device with mechanical modified electrode can significantly increase from 16.3% to 19.2%.
This study delved into the creation of a composite phase-change material (PCM), named SBNSC, for potential application in cold storage air-conditioning systems (8–16°C). The composite PCM was formulated using sodium sulfate decahydrate (Na 2 SO 4 ·10H 2 O) as the primary PCM, borax as a nucleating agent, ammonium chloride (NH 4 Cl) as a temperature control additive, sodium hexametaphosphate as a crystal modifier and carboxymethylcellulose as a thickening agent. The D-optimal mixture design was employed to optimize the composition, and this resulted in the identification of an optimal formulation characterized by the most desirable properties. The thermal properties were evaluated using differential scanning calorimetry and revealed a phase-change temperature of 15.40°C. X-ray diffraction analysis confirmed the crystalline nature of the composite PCM. After 100 cooling and heating cycles, the crystallization temperature was 9.50°C, the melting temperature was 15.50°C and the supercooling degree was 0.70°C. The enthalpy values showed an 11.22% decrease, indicating reliable thermal performance. The study concluded that the optimal formulation (SBNSC) exhibits commendable thermophysical properties, demonstrating its potential for effective application in phase-change cold storage air-conditioning systems.
Carbon-based perovskite solar cells (C-PSCs) are promising candidates for large-scale photovoltaic applications due to their theoretical low cost and high stability. However, the fabrication of high-performance C-PSCs with large-area electrodes remains challenging. In this work, we propose a novel playdough-like graphite putty as top electrode in the perovskite devices. This electrode with soft nature can form good contact with the hole-transporting layer and the conductive substrate at room temperature by a simple pressing technique, which facilitates the fabrication of both small-area devices and perovskite solar modules. In this preliminary research, the corresponding small devices and modules can achieve efficiencies of 20.29% (∼0.15 cm2) and 16.01% (∼10 cm2), respectively. Moreover, we analyze the limitations of the optical and electrical properties of this playdough-like graphite electrode on the device performance, suggesting a direction for further improvement of C-PSCs in the future.
Perovskite solar cells (PSCs) have gained momentum in the photovoltaic field because of its high-power conversion efficiency (PCE) and low-cost manufacturing process. In the screening of perovskite materials, CsPbI3 has attracted widespread due to its good thermal stability, high degradation resistance and excellent photovoltaic performance. However, CsPbI3's high band gap (1.73 eV) limits its development in single-junction PSC. In this study, we constructed a double absorption layer CsPbI3/CsGeI3 PSC with SCAPS-1D (solar cell capacitance simulator). Compared with the conventional single-layer structure of CsPbI3 PSC, the double-layer CsPbI3/CsGeI3 PSC has a broader absorption spectrum and better photovoltaic performance. We systematically analyzed the thickness and defect density of the absorber layer, and the operating temperature on the cell performance and obtained the optimal parameters. In addition, we selected the best electron transport layer and hole transport layer (ETL/HTL) for the double absorption layer CsPbI3/CsGeI3 PSC. Under the optimized conditions, all cell performance parameters improve significantly, and its PCE is as high as 28.92%.
The exciton-phonon coupling is an very important process which determines the carrier mobility in organic crystals.By making some assumptions, the Hamiltonian be validate for organic molecular crystals.In the paper, the theory of carrier transport is extended to take weak quadratic coupling into account, and the diffusion constant be attained for a simple model.From the quadratic exciton-phonon coupling, we can get the analytical solution of carrier mobility in the organic molecular crystals.Experimentally, carrier mobility can be determined by various techniques.
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